Reinforcement in Cooperative Games

Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Επιστήμη Δεδομένων και Μηχανική Μάθηση

Transferability of Insights from Fundamental Investigations into Practical Applications of Prechamber Combustion Systems

Efforts to reduce CO2 emissions from spark ignition engines have driven engine development to lean-burn or high-dilution operation, which results in high combustion variability as well as increased unburned hydrocarbon emissions. A widely used technology to reduce these issues are prechamber ignition systems, in which the external ignition source is located in a separate small volume, connected to the main chamber via small orifices. This setup allows for design of favourable ignition conditions near the ignition source, which results in fast and repeatable early flame propagation. The pressure increase resulting from combustion taking place inside the prechamber leads to the ejection of jets containing hot combustion products and possibly active radicals into the main chamber, which ignite the lean or diluted mixture; this process is often dubbed turbulent jet ignition or TJI. The use of TJI systems in engines allows the combustion of very lean/diluted mixtures, resulting in higher efficiencies and lower NOx emissions. In this work we shed light into the importance of quenching for practical applications involving turbulent jet ignition. This is achieved through optical investigations in a generic, constant volume test-rig, combined with zero-dimensional (0-D) model calculations. The 0-D model is applied to the generic setup and in real engine applications under varying operating conditions, in order to highlight the relative importance of quenching under the various thermochemical conditions encountered. The results indicate that thermal quenching in the nozzle should not be expected due to the small flame thickness under high pressure encountered in internal combustion engines. Nevertheless, under the jet mixing conditions expected in engines, hydrodynamic quenching due to mixing of burned products with unburned (cold) main chamber mixture can be expected. In most engine conditions, the re-ignition process of the initially quenched jet after their exit from the prechamber is expected to be so fast, that quenching will not be apparent in most measurements

A Novel One- and Zero-Dimensional Model for Turbulent Jet Ignition

Turbulent jet ignition (TJI) is a promising combustion technology for burning highly diluted air-fuel mixtures. Computationally efficient models to assess the effect of the operating conditions and design parameters on the ignition propensity and timing are of paramount importance for the development of combustion systems employing TJI. To this end, a one-dimensional (1-D) jet model, which is based on the solution of the section integrated mass and momentum conservation equations, is derived in the present study. The model is extended with two additional transport equations for the turbulence intensity and the ignition precursor/tracer, that marks the ignition event. One-dimensional transient flamelet calculations are performed to generate tables for the ignition precursor source term that account for the turbulence and chemistry interaction. Further simplification of the model is carried out to obtain a novel penetration correlation and a computationally inexpensive Lagrangian ignition model. The extended jet model is hierarchically validated using available literature data for non-reactive and reactive jets, as well as experiments conducted in a state-of-the-art optically accessible prechamber. The derived model is able to reproduce both flow-related quantities (velocity and turbulence profiles, jet penetration) and the ignition delay time under different variations. This study also illustrates how numerical simulations in canonical configurations (one-dimensional flamelet) can be used in practical applications of TJI.ISSN:1386-6184ISSN:1573-198